1. Field of the Invention
The present invention relates to an ink jet printing apparatus that performs a printing operation while conveying a print medium to printing means as well as a method for controlling the ink jet printing apparatus.
2. Description of the Related Art
In the prior art, printing apparatuses such as printers are often used to print photographic images. In particular, ink jet printing apparatuses can form images having quality at least equivalent to that of silver photographs owing to a reduction in the size of ink droplets and an improvement in image processing technique. In the ink jet printing apparatus, a print head is kept out of contact with print media conveyed by conveying means and ejects ink droplets. The ink droplets land on the print medium to form an image. Thus, the quality of the image is greatly affected by the accuracy with which the print medium is conveyed. In the ink jet printing apparatus, conveying means is provided upstream and downstream of the print head to convey the print medium; the print head serves as printing means. The upstream conveying means comprises a conveying roller serving as a driving roller that rotates intermittently and a pinch roller provided opposite the conveying roller and serving as a driven roller. A print medium provided by a sheet feeding section is fed downstream by sandwiching it between both rollers and rotating the rollers. Therefore, the upstream conveying means exerts a conveying force until the trailing end of the print medium passes between both rollers.
The print medium fed downstream by the upstream conveying means is printed by the print head and then intermittently conveyed further downward by the downstream conveying means. Then, the print medium is finally discharged to a discharging section.
It is assumed that during the operation of intermittently conveying the print medium, the conveying roller is intermittently stopped immediately before the trailing end of the print medium through a nip formed by the conveying roller and the pinch roller, that is, the conveying roller is stopped with the trailing end of the print medium remaining at the nip. Then, the pinch roller urged against the conveying roller may be rotated by the urging force to push out the print medium downstream from between both rollers. In this case, the print medium is conveyed by a conveying amount larger than a preset conveying pitch. As a result, the image may disadvantageously be uneven.
To eliminate this disadvantage, Japanese Patent Application Laid-Open No. 2002-254736 discloses a technique that controls the conveyance of the trailing end, while suppressing printing deviations by shifting the range of active nozzles in the print head.
This patent document mainly discloses the following:
(1) A sensor senses the position of the trailing end of the print medium so that the trailing end does not remain at the nip portion of the upstream conveying means.
(2) Nozzles located downstream in a print medium conveying direction are not driven so that an image is formed using only the upstream nozzles before the conveyance of the print medium is stopped at the position where the print medium does not remain at the nip position.
(3) The print medium is conveyed to a position downstream of the nip so that the trailing end of the print medium does not remain at the nip portion.
(4) The set of active nozzles is shifted downstream in the print medium conveying direction before an image is formed.
However, the technique disclosed in Japanese Patent Application Laid-open No. 2002-254736 has not solved the problems shown below.
(A) To convey the print medium as described in (1), the conveying amount must be increased above the preset conveying pitch. However, an increase in conveying amount may accumulatively cause a large number of conveying errors in conveying means. This reduces conveying accuracy.
Thus, with the technique disclosed in Japanese Patent Application Laid-open No. 2002-254736, the conveying accuracy must be ensured by increasing the precision of the components of the downstream conveying means, which performs a conveying operation after the print medium has passed through the upstream conveying means. This increases manufacturing costs.
(B) It is necessary to control the amount by which the print medium is conveyed when its trailing end passes through the nip of the upstream conveying means and to shift the range of active nozzles in the print head. This increases printing time compared to that required for normal printing operations. In particular, the increase in printing time is significant if marginless printing is carried out in which an image is printed all over the surface of the print medium without any margins at the ends of the print medium.